Friday, October 30, 2009

Executing JavaScript from HTML AJAX

AJAX, by definition, is known as "asynchronous JavaScript and XML". However, the use of XMLHttpRequest is not limited to XML, with JSON being a popular alternative. This post will focus on another alternative: standard HTML. Using HTML may be necessary when the HTML is already being generated server-side, with no available XML or JSON equivalent. If the response is to be viewable by the user, HTML may already be the desired result, and can avoid additional complexities of transforming XML or JSON into HTML.

Simply retrieving HTML from an AJAX request and inserting into the existing DOM of a web page is relatively easy. For my purpose, I used the .responseText attribute of the response on the XMLHttpRequest. Once the HTML string is obtained, it can be injected into an existing DOM node in the document by assigning it to the node's .innerHTML property.

Another possibility to consider is if the returned HTML is guaranteed to be valid XHTML. In this case, it may be possible to continue treating the response as XML and using the .responseXML attribute, and inject it into an existing DOM node in the document by calling the node's .importNode(…) method.

In either case, there is the complication of scripts that may be embedded within the returned HTML. These embedded scripts are not reliably executed, especially not consistently across web browsers. This is particularly complicated by Internet Explorer's handling of the <script/> tags as "NoScope" elements, which is detailed by Microsoft's John Sudds in MSDN's .innerHTML property documentation and referenced forum thread. In summary, embedded scripts should have the "defer" property set to true, and must follow a scoped element (e.g. <input type="hidden"/>) to work with IE. I have seen several pages that suggest setting the .innerHTML on a cloned node, including the above provisions for IE, then inserting back into the HTML DOM using methods such as .appendChild(…) or .replaceChild(…) to work across browsers. However, even this is not guaranteed to always work, and fails with Safari in particular.

Current Working Solution:

Rather than using a combination of browser "hacks" and/or browser detection, this solution instead recognizes and assumes that embedded scripts will not be executed automatically by the browser. After inclusion into the document, any embedded scripts are efficiently searched for and executed through JavaScript's eval(…) method. Shown below is an example / test case:

JavaScript source code:

 namespace("com.ziesemer.demos").htmlAjax = function(){
    var pub = {};
    var contentDiv, oldContent;
    var html = "<div>New 1st-level dynamic text, presumably from AJAX response.</div>"
      + "<div id=\"com.ziesemer.demos.htmlAjax.middle\">More 1st-level dynamic text that should be replaced.</div>"
      + "<div>More 1st-level dynamic text, presumably from an AJAX response.</div>"
      + "<script type=\"text/javascript\">"
      + "document.getElementById(\"com.ziesemer.demos.htmlAjax.middle\").innerHTML = "
      + "\"2nd-level dynamic text, controlled by JavaScript presumably from an AJAX response.\";"
      + "<" + "/script>"; = function(){
      contentDiv = document.getElementById("com.ziesemer.demos.htmlAjax.output");
      oldContent = contentDiv.innerHTML;
      contentDiv.innerHTML = html;
      var scripts = contentDiv.getElementsByTagName("script");
      // .text is necessary for IE.
      for(var i=0; i < scripts.length; i++){
        eval(scripts[i].innerHTML || scripts[i].text);
    pub.reset = function(){
      if(contentDiv && oldContent){
        contentDiv.innerHTML = oldContent;
    return pub;

HTML source code: (equivalent to using this page's View/Source)

<div style="border:1px solid; padding:0.5em;">
  <div><b>Dynamic test area:</b></div>
  <div id="com.ziesemer.demos.htmlAjax.output" style="border:1px solid;">
    Original static HTML text that should be replaced.
    <input type="button" value="Go!" onclick=";"/>
    <input type="button" value="Reset" onclick="com.ziesemer.demos.htmlAjax.reset();"/>
    <b>Expected result:</b> (of what the above should appear as after the &quot;Go!&quot; button is clicked)
  <div style="border:1px solid;">
    <div>New 1st-level dynamic text, presumably from AJAX response.</div>
    <div>2nd-level dynamic text, controlled by JavaScript presumably from an AJAX response.</div>
    <div>More 1st-level dynamic text, presumably from an AJAX response.</div>


This was tested on Mozilla Firefox 3.5; Internet Explorer 6, 7, and 8; Google Chrome 3; and Apple Safari 4 for Windows. If both blocks of above text don't match after clicking "Go!", you've probably found an issue.

A few things to note:

  • This does not work with externally sourced scripts, i.e., those with a "src" attribute. This should normally not be a concern, as there shouldn't be any reason why these types of scripts can't be included in the page before the AJAX call is made. However, if supporting this is necessary, YUI's Get Utility will probably prove useful.
  • As with most similar approaches, this does not work with scripts that utilize document.write(…). If document.write is used in this method, it will likely result in all existing content of the page being removed. Again, this should normally not be a concern, as use of document.write should generally be discouraged and avoided for a number of reasons. The suggested alternative is to use existing DOM elements as placeholders, as shown in my example above.
  • Note the use of the "broken" closing </script> tag. This would not be necessary if the HTML string was actually obtained externally, e.g. from an AJAX call. It is only necessary as it is contained within a JavaScript string literal (as in the above example), as excellently described by Walt Stoneburner in his blog at (2007-04-25).
  • Not really specific to this example, but the for loop around the returned "scripts" array would ideally be replaced by Array.forEach(…), e.g.:
      // .text is necessary for IE.
      eval(script.innerHTML || script.text);
    However, .forEach isn't supported until JavaScript 1.6. Firefox 3.5 supports JS 1.8.1. Chrome 1.0 and Safari 3.2 support JS 1.7. Even version 8 of IE still only supports JS 1.5!

Additional references:

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